1. Field of the Invention
This invention relates generally to semiconductor processing and, more particularly, to the heat treatment of substrates, including semiconductor wafers or flat panel displays.
2. Description of the Related Art
Reactors which can process a substrate while suspending or floating the substrate without directly mechanically contacting the substrate, e.g., by floating the substrate on gas cushions, have relatively recently been developed for semiconductor processing. These reactors may be called floating substrate reactors and such a reactor is commercially available under the trade name Levitor® from ASM International, N.V. of Bilthoven, The Netherlands.
In the Levitor® reactor, which is also described in U.S. Pat. No. 6,183,565 B1, a substrate, such as a wafer, is supported by two opposite gas flows emanating from two heated and relatively massive reactor blocks located on opposite sides of the substrate. The boundary surfaces of the reactor blocks facing the wafer are substantially flat and a small gap of less than about 1 mm is typically maintained between each block and the corresponding wafer surface. The small gap results in a particularly rapid heat transfer from the heated blocks to the wafer by conduction through the gas. The heat-up of the wafer is thus very uniform, as the wafer is not mechanically contacted during the heat treatment. In comparison, where a transport arm transports a substrate into the reactor and then continues to support the substrate during processing, mechanical contact during processing by support fingers of a transport arm results in cold spots on the wafer during heat-up at the contact positions with the support fingers, as the support fingers represent extra thermal mass that needs to be heated and that locally slows down the heat-up rate. Alternatively, where a substrate is transported to the reactor and then handed off to support pins that remain in the reactor after processing, mechanical contact during processing by those support pins results in hot spots on the wafer at the contact positions when the wafer is handed-off and contacts the support pins. Also, by floating a substrate during processing, thermal stresses, possibly resulting in crystallographic slip, are advantageously avoided.
A method utilizing a floating substrate reactor, such as the Levitor® reactor, to achieve a high degree of reproducibility in the thermal treatment for a series of substrates, which are successively treated one by one, is described in U.S. Patent Application Publication No. 2003/0027094 A1, published Feb. 6, 2003, and assigned to ASM International, N.V. In that method, the temperature is measured close to the boundary surface of a reactor block so that withdrawal of heat from the reactor block by the placement of a relatively cold substrate in the reactor is measured at that boundary surface. The reactor block is typically continuously heated and the cold substrate is placed in the vicinity of the reactor block only after the reactor block has reached a desired temperature, as measured at the boundary surface. The cold substrate typically will absorb heat and reduce the temperature of the reactor block. The substrate is then removed after processing and before the temperature of the continuously heated reactor block rises to the desired temperature again. After the temperature of the reactor block rises to the desired temperature, another substrate is placed in the vicinity of the reactor block.
An advantage of reactors such as the Levitor® reactor is that the relatively massive reactor blocks of the reactor act as thermal “fly-wheels,” resulting in a very stable temperature and reproducible performance. Ideally, for the most efficient operation, the reactor has a constant temperature set-point all the time.
Different process requirements, however, may require different treatment temperatures. From a semiconductor fabrication operation point of view and the standpoint of process efficiency, one thermal treatment reactor should be able to perform these different processes. However, changing the temperature of reactors such as the Levitor® reactor, and cooling-down the reactor, in particular, is a very time-consuming process that can negatively influence the applicability of such a reactor for performing sequences of processes requiring different process temperatures.
Accordingly, it is an object of the present invention to provide an improved method for thermally treating a substrate in a floating substrate reactor at different temperatures.